1. Field of the Invention
The present invention relates to a thin film transistor, a method of producing the thin film transistor, a liquid crystal display, and a thin film forming apparatus. More particularly, the present invention relates to the structure of a gate insulating film in a thin film transistor of the reverse stagger type.
2. Description of the Related Art
FIG. 14 relates to a conventional ordinary liquid crystal display using thin film transistors (hereinafter referred to as TFTS), and illustrates one example of the structure of a TFT array board including TFTs of the reverse stagger type, gate lines, source lines, etc. In such a TFT array board, as shown in FIG. 14, gate lines 50 and source lines 51 are arranged on a transparent substrate in a matrix pattern. Each of. areas surrounded by the gate lines 50 and the source lines 51 serves as one pixel 52, and a TFT 53 is provided for each pixel 52. FIG. 15 is a sectional view showing a construction of the TFT 53.
In the TFT 53, as shown in FIG. 15, a gate electrode 55 leading out of the gate line 50 is formed on a transparent substrate 54, and a gate insulating film 56 is formed in covering relation to the gate electrode 55. A semiconductor active film 57 made of amorphous silicon (a-Si) is formed on the gate insulating film 56 at a position above the gate electrode 55. A source electrode 59 leads out of the source line 51 and a drain electrode 60 are formed to extend over the semiconductor active film 57 through an ohmic contact layer 58 which is made of amorphous silicon (a-Si:n+) containing an n-type impurity such as phosphorous, and then on the gate insulating film 56. A passivation film 61 is formed in covering relation to the TFT 53 made up of the source electrode 59, the drain electrode 60, the gate electrode 55, etc., and a contact hole 62 is formed in the passivation film 61 at a position above the drain electrode 60. Further, a pixel electrode 63 formed of a transparent conductive film, such as indium tin oxide (hereinafter referred to as ITO), is filled in the contact hole 62 for electrical connection to the drain electrode 60.
Of the components of the TFT thus constructed, the gate insulating film located between the gate electrode and the semiconductor active film is the most important component that dominates electrical characteristics and reliability of the TFT. Also, the gate insulating film is an element that is responsible for the occurrence of surface defects. For an amorphous-silicon TFT using amorphous silicon as a material of the semiconductor active film, a redundant structure endurable against defects has been tried by employing a two-layered gate insulating film structure wherein gate insulating films are formed as two stacked layers using different materials and different methods. In one example of such a structure, the two stacked layers are a dense film of Ta2O5 formed by anode-oxidizing tantalum (Ta) of the gate electrode and a film of Si3N4 deposited by the plasma CVD.
Regarding electrical characteristics of the TFT, generally-demanded capabilities of the gate insulating film are represented by a dielectric withstand voltage and a carrier mobility in the semiconductor active film. The dielectric withstand voltage is a problem inherently depending on the gate insulating film itself, whereas the carrier mobility in the semiconductor active film is affected by an interface characteristic between the gate insulating film and the semiconductor active film.
The term xe2x80x9cdielectric withstand voltagexe2x80x9d means a maximum voltage until which the gate insulating film is endurable against dielectric breakdown in a test wherein the voltage applied between the gate electrode and the semiconductor active film is increased gradually. If the dielectric withstand voltage is lower than a desired design value, the gate insulating film would be liable to break down, thus resulting in an operation failure of the TFT and hence a display failure.
Also, the term xe2x80x9cmobilityxe2x80x9d means an index indicating easiness in movement of carries within the TFT. A larger value of the mobility represents a greater driving ability and a higher-speed operation of the TFT. The mobility lowers if traveling of carriers is impeded due to disorder of a semiconductor crystal and the presence of impurities. Taking electrons in silicon as an example, the mobility of electrons is about 1000 cm2/Vxc2x7sec in a single crystal. However, the mobility lowers down to the order of 0-100 cm2/Vxc2x7sec in polycrystalline silicon, and further down to the order of 0.3-1 cm2/Vxc2x7sec in amorphous silicon. In other words, because the mobility lowers in the case of using amorphous silicon due to the inherent property, there has been a demand for maintaining the mobility as high as possible even to a small extent in such a case.
Although the dielectric withstand voltage and the carrier mobility are, as described above, important factors in achieving TFTs with good electrical characteristics and high reliability, the materials which have been usually employed for the gate insulating film in the past are not satisfactory from points of both the dielectric withstand voltage and the carrier mobility. Also, although it has been hitherto proposed to combine two kinds of layers for giving the gate insulating film desired capabilities like the above-mentioned example of the two-layered structure of Ta2O5 and Si3N4, this method has such problems that the step of forming the gate insulating film is complicated and the productivity of TFT array boards is deteriorated.
With the view of solving the problems set forth above, an object of the present invention is to provide a TFT having a gate insulating film which has a high dielectric withstand voltage and can ensure a desired carrier mobility in an adjacent semiconductor active film, a method of producing the TFT, a liquid crystal display which is superior in electrical characteristics and yield, as well as a thin film forming apparatus adaptable in the method of producing the TFT.
To achieve the above object, in the TFT of the present invention, a gate electrode and a semiconductor active film are formed on a substrate with a gate insulating film, which is formed of two layered insulating films, located therebetween, the gate insulating film being made up of a first gate insulating film which is disposed on the same side as the gate electrode and improves a withstand voltage between the gate electrode and the semiconductor active film, and a second gate insulating film which is disposed on the same side as the semiconductor active film and improves an interface characteristic between the gate insulating film and the semiconductor active film.
In other words, the TFT of the present invention intends to realize a gate insulating film which has in itself a desired dielectric withstand voltage and renders the semiconductor active film to have a desired carrier mobility, by forming the gate insulating film with two layered insulating films made of such materials as functioning respectively to improve the withstand voltage between the gate electrode and the semiconductor active film, and to improve the interface characteristic between the gate insulating film and the semiconductor active film. The phrase xe2x80x9cimprove the interface characteristic between the gate insulating film and the semiconductor active filmxe2x80x9d used herein means that the carrier mobility in the semiconductor active film is improved as a result of forming the second gate insulating film.
Concrete examples of the materials usable as the first and second gate insulating films are as follows. The first and second gate insulating films are each formed of a silicon nitride film, the optical band gap of the first gate insulating film has a value in the range of 3.0 to 4.5 eV, and the optical band gap of the second gate insulating film has a value in the range of 5.0 to 5.3 eV.
Heretofore, it has been customary that an insulating film having two different functions is formed of two layered films by using two different kinds of materials. However, the inventor has found that even films having the same composition, i.e., silicon nitride films, develop different characteristics if values of the optical band gap of the films are different from each other, and has accomplished the present invention based on the finding. The correlation between concrete numerical values of the optical band gap and characteristics of the gate insulating film will be described later in Examples.
A method of producing the TFT of the present invention, which includes a process of forming the gate insulating film having the above features, comprises the steps of preparing a plasma CVD apparatus including a radio-frequency electrode and a susceptor electrode disposed in opposed relation and installed in a film forming chamber; bringing a gas mixture of silane gas and ammonia gas into a plasma state under a desired radio-frequency electric field formed between the radio-frequency electrode and the susceptor electrode, thereby forming a first gate insulating film on a gate electrode formed on a substrate; bringing a gas mixture having the same composition as the above gas mixture into a plasma state under a greater radio-frequency electric field than the above radio-frequency electric field, thereby forming a second gate insulating film on the first gate insulating film; and forming a semiconductor active film on the second gate insulating film.
When the above method is employed, radio-frequency powers can be applied to the radio-frequency electrode and the susceptor electrode of the plasma CVD apparatus in any sequence of the following combinations. It is to be here noted that the power applied to the radio-frequency electrode is called excitation power and the power applied to the susceptor electrode is called substrate bias power.
(1) The substrate bias power is not applied both in forming the first gate insulating film and in forming the second gate insulating film, whereas the excitation power is set to be greater in forming the second gate insulating film than in forming the first gate insulating film.
(2) The equal substrate bias power is applied both in forming the first gate insulating film and in forming the second gate insulating film, whereas the excitation power is set to be greater in forming the second gate insulating film than in forming the first gate insulating film.
(3) The substrate bias power is set to be greater in forming the second gate insulating film than in forming the first gate insulating film, whereas the equal excitation power is applied both in forming the first gate insulating film than in forming the second gate insulating film.
(4) The substrate bias power is set to be greater in forming the second gate insulating film than in forming the first gate insulating film, whereas the excitation power is set to be greater in forming the second gate insulating film than in forming the first gate insulating film.
Of the above combinations, the cases of applying both the substrate bias power and the excitation power require the use of a two-frequency excitation plasma CVD apparatus.
Instead of the above method of using a gas mixture having the same composition as the gas mixture used in forming the first gate insulating film and forming the second gate insulating film under a greater radio-frequency electric field than in forming the first gate insulating film, the method of producing the TFT may comprise the steps of forming the first gate insulating film in the same manner as in the above method; bringing a gas mixture, in which silane gas and ammonia gas are mixed at such a mixing ratio as containing the ammonia gas at a greater proportion relative to the silane gas than in the mixture gas used in the preceding step, into a plasma state under a radio-frequency electric field having the same intensity as the radio-frequency electric field applied in forming the first gate insulating film, thereby forming a second gate insulating film on the first gate insulating film; and forming a semiconductor active film on the second gate insulating film.
Also, when the above method of changing over the composition of the gas mixture is employed, radio-frequency powers can be applied to the radio-frequency electrode and the susceptor electrode of the plasma CVD apparatus in any sequence of the above combinations (1) to (4).
With any of the above methods, the two layered gate insulating films having different characteristics can be formed successively and easily just by forming both the first and second gate insulating films of silicon nitride films, and changing over the radio-frequency power or the composition of the gas mixture at the time of shift from the first gate insulating film forming step to the second gate insulating film forming step. As a result, the step of forming the gate insulating film is not so complicated as the example of the two-layered structure of Ta2O5 and Si3N4 mentioned above in connection with the related art, and a TFT array board can be manufactured with productivity at a level not so lower than the case of forming a one-layer gate insulating film.
In the liquid crystal display of the present invention, a liquid crystal is held between a pair of substrates disposed in opposed relation, and one of the pair of substrates includes the above TFT.
The liquid crystal display of the present invention employs a TFT array board having the TFTs which provide a high dielectric withstand voltage between the gate electrode and the semiconductor active film, and ensure a great carrier mobility in the semiconductor active film. Therefore, a liquid crystal display can be realized which has a high response speed and is superior in yield and reliability.
The thin film forming apparatus of the present invention comprises a susceptor electrode disposed in opposed relation to a radio-frequency electrode and installed in a film forming chamber for supporting a substrate thereon, and a control unit for successively carrying out the steps of supplying a reactive gas to an inner space of the film forming chamber while exhausting the gas so as to maintain a desired pressure within the film forming chamber, and bringing the reactive gas into a plasma state under a first radio-frequency electric field formed between the radio-frequency electrode and the susceptor electrode, thereby forming a first coating on the substrate; and bringing the reactive gas into a plasma state under a greater second radio-frequency electric field than the first radio-frequency electric field while maintaining the plasma state between the radio-frequency electrode and the susceptor electrode, thereby forming a second coating on the surface of the first coating.
As means for producing the second radio-frequency electric field greater than the first radio-frequency electric field, second substrate bias power applied to the susceptor electrode in forming the second coating may be set to be greater than first substrate bias power applied to the susceptor electrode in forming the first coating, while desired plasma excitation power is applied to the radio-frequency electrode. Alternatively, second plasma excitation power applied to the radio-frequency electrode in forming the second coating may be set to be greater than first plasma excitation power applied to the radio-frequency electrode in forming the first coating.
Another thin film forming apparatus of the present invention comprises a susceptor electrode disposed in opposed relation to a radio-frequency electrode and installed in a film forming chamber for supporting a substrate thereon, and a control unit for successively carrying out the steps of supplying a first gas mixture of monosilane gas and ammonia gas, which are mixed at a first mixing ratio, to an inner space of the film forming chamber while exhausting the gas so as to maintain a desired pressure within the film forming chamber; bringing the first gas mixture into a plasma state under a radio-frequency electric field formed between the radio-frequency electrode and the susceptor electrode, thereby forming a first silicon nitride film on the substrate; and supplying a second gas mixture of monosilane gas and ammonia gas, which are mixed at such a second mixing ratio as containing the ammonia gas at a greater proportion than at the first mixing ratio, to the inner space of the film forming chamber while maintaining the plasma state between the radio-frequency electrode and the susceptor electrode, and bringing the second gas mixture into a plasma state to thereby form a second silicon nitride film on the surface of the first silicon nitride film.
With the thin film forming apparatus of the present invention, the two layered coatings having different characteristics can be formed successively in the single apparatus.